Black holes are areas of maximal local entropy; that is, they have maximum entropy for the (nominal) volume they occupy. Why? Because they are absorbing matter or energy that has a finite amount of entropy but compressing it into a fundamentally minimal volume (as compact as it is possible for energy to exist). This is in conflict with a classical view of thermodynamics, in which the absorption of all associated matter and energy is complete, i.e. there is “no hair”, no way to tell what is in within the event horizon of a black hole because no particles (and thus, no information) can escape, and no low temperature reservoir to which to provide entropy balance. This is resolved by permitting a black hole to radiate away a certain amount of energy (from which comes the quantum electrodynamic field theory-based “Hawking radiation”) at a specific temperature based strictly on its mass, electric charge, and rotational momentum, so that it can maintain thermodynamic balance and still behave as General Relativity describes. (A black hole can essentially be treated as single a giant composite quantum particle insofar as it can be distinguished only by these limited properties.) Stellar mass black holes actually radiate energy at a lower temperature than the surrounding background, and so are themselves electromagnetically invisible from any distance, though the behavior of matter surrounding and infalling into them can give signiture traces in the form of synchnotron radiation.
Actually, homogeneity of energy is characteristic of a high entropy state. The early universe enjoyed low entropy by being so compressed in volume–and with such a vast amount of unbound energy–that even very tiny anisotropies gave low entropy. As the universe expanded symmetries between the fundamental forces broke, and as matter and energy condensed into observable forms these fine differences led to large variations in thermal and gravitational equilibrium, which in turn formed the non-fundamental celestial structures (from stars to galactic superclusters) we see today, and eventually, higher elements and rocky bodies like the Earth.
Although it is common to think of the early universe as being a big singularity, the fact is that the space it occupied was also singular; unlike a black hole, it had no external place to radiate to. The model of a black hole is then turned on its head, as if the universe is a black hole in reverse or inside out. Could we be inside of an expanding black hole which was conceived at the Big Bang? It has been suggested, though by no means widely accepted or falsifiable, but the implications are pretty meaningless, even for theorists. Just as we cannot see into the event horizon of a black hole, the occupants of a black hole could not see outside or obtain useful information across that boundary. We couldn’t even tell if the basic physical constants, and thus the behavior and extent of force interactions, would be the same.
Dark energy is basically a placeholder for a phenomenon which mathematically satisfies empirical observations but has no real explanation. We know, or at least think that we know, that there is a repulsive action that works upon spacetime in opposition to normal gravity, either as a previously unknown long range component of gravity, or some other fundamental force interaction that causes spacetime to expand. We don’t actually know anything about the underlying nature of it, though, only that it is needed in some form to copy with the Hubble Flow and to give instantiation to a stable or accelerating expanding universe.
Personally, I find the term “dark energy” to be far to prosaic and prefer “crepuscular palpitation” instead. Plus, then people look at me funny, like I’m describing an unspeakable medical procedure, which is a great way to keep a couch to yourself at a crowded party.
Stranger